Ammonite zones of northern Chihuahua - New Mexico Geological ...

98 NEW MEXICO GEOLOGICAL SOCIETY-TWENTIETH FIELD CONFERENCE
W. T. Keller had a group of Swiss geologists in northern
Chihuahua, working for "El Aguila" (La Compania de
Méxicana de Shell ). Keller collected Campanian fossils in
the vicinity of Ojinaga and La Mula, and Jurassic fossils at
San Sostenes. These fossils were taken to Tampico and were
eventually deposited in the University of Texas collections
as the "El Aguila" part of the W. S. Adkins collection.
These fossils had also been seen by Burkhardt (1933) and
still accompany a label bearing the name of the collector,
the date, and the locality.
REMARKS ON
AMMONITE ZONATION
The ammonite zones of Tables 1 and 2 are incomplete.
There are three reasons for incomplete ammonite zonation:
(1) collection failure, (2) wrong environment, e. g., ammonites
are not found in non-marine rocks, nor are they
found in rudistid-bearing strata, and ( 3) lacunae. The first
two examples appear to be mainly responsible for the
absence of many ammonite zones in northern Chihuahua.
JURASSIC ZONES
I have listed only two Jurassic zones from northern Chihuahua.
W. S. Adkins and R. E. King collected the fossils,
and Imlay (1943) described them. However, there may be
more than two zones, as indicated by the age comparisons
of Imlay (1943, p. 529) .
The absence of Upper Kimmeridgian and Lower Tithonian
zones in the Placer de Guadalupe area is probably
the result of collection failure in rocks that do not abound
with fossils. Upper Tithonian and Lower Neocomian
(Berriasian and Valanginian zones) are likwise probably
absent by collection failure in this vicinity; they could be
cut out by complex structure, or these slightly fossiliferous
beds may not as yet have yielded fossils. No one has found
a Jurassic-Cretaceous unconformity in this area, but I must
admit that not many have tried. Fossils of the Kossmatia
rancheriaensis Imlay zone also occur in the Jurassic Malone
Formation of the Malone Mountains; the specimen that I
have identified is a Kossmatia kingi Imlay. Otherwise, the
Jurassic is not known north or northeast of the vicinity of
Placer de Guadalupe, unless the Jurassic at Sierra de Ojo
Caliente, stop 4 of trip A-13, 20th International Geological
Congress (Diaz, 1956, p. 312, Fig. 4) has been correctly
dated.
CRETACEOUS ZONES
The collection in the summer of 1968 by D. S. Webb of
a good specimen of Leopoldia victoriensis Imlay (1938)
from the sandstones along the highway south of Samalayuca
has changed the age of these rocks, formerly reported as
Jurassic by William E. Humphrey (Diaz, 1956, p. 28) on
field identification only. These rocks are lower Hauterivian.
The same rocks with the same fossils occur at Placer de
Guadalupe where Bridges (1962, 1965, pp. 74-76) reported
them as Jurassic because I made misidentifications similar
to Humphrey's. An isolated specimen, in different lithology,
of Leopoldia sp. cf. leopoldinus (d'Orbigny), collected by
DeFord from near the picachos in the vicinity of Placer de
Guadalupe, likewise is lower Hauterivian. Two Lower Cretaceous
ammonites (presumably pre-Aptian) are rumored
to have come from near Cuchillo Parado, but these seem to
be misplaced. No pre-Aptian ammonites from northern
Chihuahua, other than those mentioned above, are known
to the writer.
Jurassic and Cretaceous fossils are reported at Sierra de
Ojo Caliente (Diaz, 1956, p. 26, 31, 32 ). I have seen no
identifiable fossils from this locality.
The absence of upper Hauterivian, Barremian, and most
of the Lower Aptian ammonites from northern Chihuahua
can usually be attributed to the occurrence of the wrong
facies. In many localities the Navarette evaporite section is
overlain by continental deposits, as on the west flank of the
Sierra de la Aldea, east of Cuchillo Parado. At this locality,
above the evaporites, pulmonate land snails have been collected
by Young, Powell, Diaz, and DeFord. In the southern
Quitman Mountains, dinosaur bones were collected by
Donald Campbell (1968) and Bill Jones (1968) from the
Las Vigas formation. How much of the Las Vigas Formation
is terrestrial is debatable, but the Las Vigas and Navarette
formations certainly represent environments inhospitable
to ammonites.
The first ammonites above the lower Hauterivian represent
the latest Lower Aptian. These are Cheloniceras sp.,
one specimen from the Sierra del Presidio; Cheloniceras
adkinsi Scott, from the southern Quitman Mountains,
which may not be a Cheloniceras; and a specimen of Cheloniceras
from just above the redbed sequence on the east
flank of Sierra Cuervo, north of Aldama. Only in these
three areas have probable late Lower Aptian fossils been
collected.
Upper Aptian fossils are well known in northern Chihuahua
and adjacent Trans-Pecos Texas (Table 1).
The zone of Dufrenoyia justinae (Hill) is not as widespread
as heretofore assumed, since myself and others have
misidentified Hypacanthoplites bakeri Scott as D. justinae
on more than one occasion. In addition, the Dufrenovia
justinae zone may represent other zones too, but the geological
sections have not been collected in detail, and none
have received the type of study necessary to produce exact
collections.
Overlying the zone of Dufrenoyia justinae in the southern
Quitman Mountains of Trans-Pecos Texas, the Sierra
del Presidio, and the Bisbee district of southeastern Arizona
is the zone of Kazanskyella trinitense (Scott). Finer collecting
may also produce a more detailed zonation at this level.
This fauna has not been collected to the south of these
areas, probably because it is replaced by the late Aptianlower
Albian rudistid complex. Gillerman (1958, pp. 47-50)
collected a fauna of approximately the same age from the
Carbonate Hill Limestone in the central Peloncillo Mountains.
Some of his specimens seem to be closely related to
Kazanskyella n. sp. from the Bexar Shale in the Spahn well
in Gonzales County, south Texas. I also have a good specimen
of Kazanskyella trinitense (Scott) from the Sierra de
Sabinas-Lampazos, northwest of Sabinas Hidalgo, Nuevo
Leon.
The Lower Albian zone of Hypacanthoplites bakeri Scott